Determining The Condition of My Marine Engine (Part 2)

Topic: Boat Repowering (Part 2)

Question #1: My 2010 Model Mercruiser SR 383 Mag Marine Engine Feels Down on Power. How Can I Check The Condition of the Engine?

Response: The quick and easy answer is to perform a compression test, however this is not always the best approach as it only paints a portion of the picture to the overall condition of marine engines. We recommend performing a compression test as well as a cylinder leak down test.

Findings: After performing a quick compression test we noticed cylinder #6 registered 0 psi compression. Performing a leak down test we determined this cylinder had significant leak down through the valvetrain. This was determined by verifying that the leakage was passing through and out the intake manifold. The leak down test also revealed that although cylinder #5 had a good compression reading, it also excessive leakage. This leakage was verified to be passing through the crankcase which indicated the leakage was passing by the rings.

Conclusion: The engine run time below indicating that the engine was not run over 4,000 rpm suggested the boat was not propped according to the manufactures specification of 4,800 – 5,200 rpm. Valve “tuliping” is a sure sign of a boat being over-propped. An overloaded engine will cause excessive cylinder pressures and heat, which typically results in the intake valve deforming and losing its seat/seal. NOTE: THIS IS THE DIRECT RESULT FROM RUNNING TOO LARGE OF A PROPELLER. READ MORE on how to select the correct propeller.

Helpful Hints:
Due to standard engine clearances and normal wear, no cylinder should be expected to maintain a perfect no-leak reading. It is important that all cylinders have a relatively constant reading both with regards to a compression and leak down tests. A difference of 5 psi leak down between cylinders and +/- 10% compression is satisfactory. A difference of 10-15 psi leak down indicates the need for further investigation.

By listening or looking for the following, the cause of low pressure can be determined: